In recent years, surface-treated steel sheets to which anticorrosiveness is imparted, particularly hot-dip galvanized steel sheets and hot-dip galvannealed steel sheets, are widely used as materials of components etc. used in the fields of automobiles, home appliances, construction materials, etc. From the viewpoint of improvements in automotive fuel efficiency and automotive crash safety, there is an increasing demand for a higher-strength automobile body material. Such a material allows a reduction in wall thickness of an automobile body, and the weight of the automobile body itself can thereby be reduced, and its strength can also be increased. Therefore, the application of such a high-strength steel sheet to automobiles is being promoted.
Generally, a steel sheet used as the base material of a hot-dip galvanized steel sheet is a thin steel sheet obtained by subjecting a slab to hot rolling or cold rolling. The high-strength hot-dip galvanized steel sheet is produced by recrystallization annealing in an annealing furnace in a continuous hot-dip galvanizing line (hereafter referred to as a CGL) and hot-dip galvanizing treatment in a coating apparatus in the CGL. A hot-dip galvannealed steel sheet is produced by performing alloying treatment after the hot-dip galvanizing treatment.
Examples of the heating furnace type of the annealing furnace in the CGL include a DFF (direct fired furnace) type, an NOF type (non-oxidizing furnace type), and an all-radiant tube type. In recent years, the construction of CGLs quipped with all-radiant tube type heating furnaces is increasing because of ease of operation, infrequent occurrence of pickup, etc., which make it possible to produce high-quality coated steel sheets at low cost. However, unlike the DFF (direct fired furnace) type and the NOF (non-oxidizing furnace) type, the all-radiant tube-type furnace includes no oxidizing step immediately before annealing. Therefore, the all-radiant tube-type furnace is disadvantageous in terms of ensuring coatability when a steel sheet containing easily oxidizable elements such as Si and Mn is treated using a facility including this heating furnace.
Patent Literature 1 discloses, as a method for producing a hot-dip galvanized steel sheet using as a base material a high-strength steel sheet containing large amounts of Si and Mn, a technique in which annealing is performed at recrystallization temperature to 900° C. and then a coating is formed. Patent Literature 2 discloses a technique in which annealing is performed at 750 to 900° C. and then a coating is formed. Patent Literature 3 discloses a technique in which annealing is performed at 800 to 850° C. and then a coating is formed. When a high-strength steel sheet containing large amounts of Si and Mn is annealed at a high temperature higher than 750° C., Si and Mn in the steel are selectively oxidized and form oxides on the surface of the steel sheet. This may cause deterioration in the adhesion of the coating and the occurrence of defects such as bare spots.
Patent Literature 4 and Patent Literature 5 disclose a technique in which heating temperature in a reducing furnace is specified using a formula represented by the partial pressure of water vapor to increase dew point to thereby internally oxidize the surface layer of base steel. In the techniques described above, since the dew point is controlled over the entire area of the furnace, the dew point is difficult to control, and stable operation is difficult to achieve. When a hot-dip galvannealed steel sheet is produced with the dew point controlled unstably, internal oxides formed in the base steel sheet are distributed non-uniformly. The non-uniform distribution of the internal oxides may cause defects such as uneven coating wettability and uneven alloying in the longitudinal and transversal directions of the steel sheet.
Patent Literature 6 discloses a technique in which not only the concentrations of H2O and O2, which are oxidizing gases, but also the concentration of CO2 is specified to internally oxidize the surface layer of base steel and suppress external oxidation to thereby improve coating appearance. However, in Patent Literature 6, the presence of the internal oxides causes cracking to occur easily during forming, and the resistance to coating delamination deteriorates. With the technique in Patent Literature 6, deterioration in corrosion resistance also occurs. In addition, CO2 causes pollution of the furnace, carburization of the steel sheet surface, etc., and this may cause a problem in that mechanical properties change.
Recently, high-strength hot-dip galvanized steel sheets and high-strength hot-dip galvannealed steel sheets are increasingly used for portions that are subjected to severe forming, and importance is placed on resistance to coating delamination during severe forming. Specifically, when a coated steel sheet is bent more than 90° to form an acute angle or is subjected to forming by applying an impact to the steel sheet, there is a need to suppress the occurrence of delamination of the coating on the portion subjected to forming.
To meet the above-described characteristics, it is necessary not only to add a large amount of Si to the steel to ensure a desired steel sheet structure but also to control the texture and structure of the surface layer of the base steel immediately below the coating layer in a more sophisticated manner because cracks may start from the surface layer during severe forming. However, it is difficult to achieve the control of the above texture and structure using the conventional techniques. Specifically, with the conventional techniques, it is not possible to produce a hot-dip galvanized steel sheet excellent in resistance to coating delamination during severe forming using, as a base material, a high-strength Si-containing steel sheet in a CGL provided with an all-radiant tube-type heating furnace used as an annealing furnace.